The invention relates to a wellhead assembly for suspending casing strings concentrically into a well such as an oil well or a gas well.
A well comprises a borehole drilled from surface to an underground formation layer from which oil and/or gas is produced. Such a well is cased to prevent collapse of the borehole. A well is started with a relatively large hole which is cased as soon as possible with a so-called surface casing or conductor which is cemented in the borehole. The well is then continued with a smaller hole drilled to an intermediate depth; this hole is cased by an intermediate casing which extends to surface. The upper end of the intermediate casing extends concentrically through the conductor and at least part of the intermediate casing is cemented in the borehole. The well is then continued with again a smaller hole drilled to the production zone; this hole is cased by the production casing which extends to surface. The upper end of the production casing extends concentrically through the intermediate casing and at least part of the production casing is cemented in the borehole.
The upper ends of the intermediate casing and the production casing are suspended from a wellhead assembly which is supported on the surface casing.
A conventional wellhead assembly comprises a housing supported on the surface casing. The housing is provided with a central bore which is provided with internal support shoulders. The wellhead assembly further includes casing hangers for suspending the intermediate casing and the production casing from the internal support shoulders in the housing, and sealing means for sealing off the annular spaces around the casing hangers.
Access to the sealing means may be required, for example if leakage from an annular space occurs. A major problem with this kind of wellhead is that the sealing elements cannot be accessed without cutting one or more casings.